nanostructured and conventional (HOSP) YSZ powder particles were thermally sprayed via APS. The produced coatings were heat-treated in air at 1400 ◦C for 1, 5 and 20 h. The microstructural characteristics, porosity, thermal diffusivity and elastic modulus values of the as-sprayed and heat-treated coatings were evaluated. The main conclusions are the following:
The nanostructured YSZ coating was engineered to exhibit a bimodal distribution in its microstructure, which was formed by (i) previously molten and resolidified YSZ particles (matrix) that surrounded (ii) previously semi-molten porous nano YSZ agglomerates (nanozones) that were embedded in the coating microstructure during thermal spraying. The percentage of nanozones embedded in the coating structure was approximately 35%.
The coarse porosity levels of the nanostructured coating increased during the heat treatment. After 20 h at 1400 ◦C, the coarse porosity level of the nanostructured coating was 3.5 times higher than that of the conventional one.
This uncommon behaviour occurs due to the different sintering rates of the matrix and nanozones that form the nanostructured coating. The nanozones, due to their higher surface area, exhibit a higher driving force for sintering, thereby shrinking at much faster rates than those of the matrix and creating voids (coarse pores) within the coating structure upon high temperature exposure.
The as-sprayed thermal diffusivity value of the conventional coating is ∼60% higher than that of the nanostructured one, a difference that is maintained even after a heat treatment at 1400 ◦C for 20 h. The relative elastic modulus value of the conventional coating after 20 h at 1400 ◦C is significantly higher than that of the nanostructured one. These effects are highly associated with the porosity evolution in both coatings.
The nanostructured YSZ coating developed in this study reacts to the high temperature environment in such a way that differential sintering rates prevent steep continuous increase of thermal diffusivity and elastic modulus values over time. This study demonstrates that nanostructured YSZ coatings can be engineered to counteract sintering effects and exhibit significantly lower increases in thermal diffusivity and elastic modulus values in high temperature environments when compared to those of conventional YSZ coatings. A series of other tests are planned to verify the effectiveness of these nano YSZ coatings for TBC applications; however, the results reported in this study are considered very promising.
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